U.S. patent number 5,961,362 [Application Number 08/927,367] was granted by the patent office on 1999-10-05 for method for in situ cleaning of electron emitters in a field emission device.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Babu Chalamala, Thomas L. Credelle, Arthur J. Ingle, Charles Rowell.
United States Patent |
5,961,362 |
Chalamala , et al. |
October 5, 1999 |
Method for in situ cleaning of electron emitters in a field
emission device
Abstract
A method for in situ cleaning of electron emitters (126, 226,
326, 526) in a field emission device (100, 200, 300, 400, 500)
includes the steps of controllably providing hydrogen gas (142,
242, 342, 542) within the field emission device (100, 200, 300,
400, 500) at a time during the operational life of the field
emission device (100, 200, 300, 400, 500) and, thereafter, emitting
electrons from the electron emitters (126, 226, 326, 526), thereby
forming hydrogen free radicals, which decontaminate and condition
the emissive surfaces of the electron emitters (126, 226, 326,
526).
Inventors: |
Chalamala; Babu (Chandler,
AZ), Ingle; Arthur J. (Chandler, AZ), Rowell; Charles
(Tempe, AZ), Credelle; Thomas L. (Phoenix, AZ) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
25454644 |
Appl.
No.: |
08/927,367 |
Filed: |
September 9, 1997 |
Current U.S.
Class: |
445/59 |
Current CPC
Class: |
H01J
9/025 (20130101) |
Current International
Class: |
H01J
9/02 (20060101); H01J 009/38 () |
Field of
Search: |
;445/59,9,10,16
;313/309,336,351,495 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Patel; Nimeshkumar D.
Assistant Examiner: Smith; Michael J.
Attorney, Agent or Firm: Pickens; S. Kevin
Claims
We claim:
1. A method for in situ cleaning of electron emitters in a field
emission device comprising the steps of:
introducing hydrogen gas into the field emission device at a time
subsequent to sealing of the field emission device; and
thereafter, forming hydrogen free radicals from the hydrogen
gas.
2. The method for in situ cleaning of electron emitters as claimed
in claim 1, wherein the step of introducing hydrogen gas into the
field emission device includes the step of controllably introducing
hydrogen gas within the field emission device.
3. The method for in situ cleaning of electron emitters as claimed
in claim 2, wherein the step of controllably introducing hydrogen
gas into the field emission device includes the step of
periodically introducing hydrogen gas within the field emission
device.
4. The method for in situ cleaning of electron emitters as claimed
in claim 1, wherein the step of forming hydrogen free radicals from
the hydrogen gas includes the step of emitting electrons from the
electron emitters.
5. The method for in situ cleaning of electron emitters as claimed
in claim 1, wherein the step of introducing hydrogen gas into the
field emission device includes the step of diffusing hydrogen gas
through a hydrogen-selective membrane.
6. The method for in situ cleaning of electron emitters as claimed
in claim 5, wherein the step of diffusing hydrogen gas through a
hydrogen-selective membrane includes the step of diffusing hydrogen
gas through a hydrogen-selective membrane disposed in registration
with a hole defined by the device package.
7. A method for in situ cleaning of electron emitters in a field
emission device having an evacuated interspace region, the method
comprising the steps of:
providing a hydrogen source within the evacuated interspace region
of the field emission device;
releasing hydrogen gas from the hydrogen source; and
thereafter, forming hydrogen free radicals from the hydrogen
gas.
8. The method for in situ cleaning of electron emitters as claimed
in claim 7, wherein the step of providing a hydrogen source
includes the steps of providing a member made from a refractory
metal, heating the member in a hydrogen atmosphere, and thereafter
cooling the member, thereby entrapping hydrogen within the
member.
9. The method for in situ cleaning of electron emitters as claimed
in claim 8, wherein the step of providing a member made from a
refractory metal includes the step of providing a member made from
palladium.
10. The method for in situ cleaning of electron emitters as claimed
in claim 7, wherein the step of releasing hydrogen gas from the
hydrogen source includes the step of impinging electrons onto the
hydrogen source.
11. The method for in situ cleaning of electron emitters as claimed
in claim 10, wherein the step of impinging electrons onto the
hydrogen source includes the step of impinging onto the hydrogen
source electrons emitted by at least one of the electron
emitters.
12. The method for in situ cleaning of electron emitters as claimed
in claim 7, wherein the step of releasing hydrogen gas from the
hydrogen source includes the step of heating the hydrogen
source.
13. The method for in situ cleaning of electron emitters as claimed
in claim 7, wherein the step of forming hydrogen free radicals from
the hydrogen gas includes the step of emitting electrons from the
electron emitters.
14. The method for in situ cleaning of electron emitters as claimed
in claim 7, wherein the step of releasing hydrogen gas from the
hydrogen source includes the step of controllably releasing
hydrogen gas from the hydrogen source.
15. The method for in situ cleaning of electron emitters as claimed
in claim 14, wherein the step of controllably releasing hydrogen
gas from the hydrogen source includes the step of releasing
hydrogen gas from the hydrogen source in response to activation of
the field emission device.
16. The method for in situ cleaning of electron emitters as claimed
in claim 14, wherein the step of controllably releasing hydrogen
gas from the hydrogen source includes the step of periodically
releasing hydrogen gas from the hydrogen source.
17. The method for in situ cleaning of electron emitters as claimed
in claim 14, wherein the step of controllably releasing hydrogen
gas from the hydrogen source includes the step of releasing
hydrogen gas from the hydrogen source in response to the drop below
a predetermined value of a test emission current measured at a test
electron emitter in the field emission device.
18. The method for in situ cleaning of electron emitters as claimed
in claim 14, wherein the step of controllably releasing hydrogen
gas from the hydrogen source includes the step of releasing
hydrogen gas from the hydrogen source at a rate sufficient to clean
the electron emitters and thereby maintain stable electron emission
for a given set of conditions within the field emission device.
19. A method for in situ cleaning of electron emitters in a field
emission device comprising the steps of:
sealing the field emission device; and
thereafter, adjusting a partial pressure of hydrogen gas within the
field emission device in a manner sufficient to maintain stable
electron emission having fluctuations within a tolerable range.
20. The method for in situ cleaning of electron emitters as claimed
in claim 19, wherein the step of adjusting a partial pressure of
hydrogen gas includes the step of maintaining within the field
emission device a partial pressure of hydrogen greater than
10.sup.-8 Torr.
21. The method for in situ cleaning of electron emitters as claimed
in claim 20, wherein the step of adjusting a partial pressure of
hydrogen gas includes the step of maintaining within the field
emission device a partial pressure of hydrogen within a range of
10.sup.-8 to 10.sup.-5 Torr.
Description
REFERENCE TO RELATED APPLICATION
Related subject matter is disclosed in a co-pending, commonly
assigned patent application entitled "Field Emission Device Having
Means for in Situ Feeding of Hydrogen", attorney docket number
FD97060, filed on even date herewith.
FIELD OF THE INVENTION
The present invention pertains to the area of field emission
devices and, more particularly, to methods for cleaning and
conditioning electron emitters in a field emission device.
BACKGROUND OF THE INVENTION
A typical field emission device contains electron emitters, such as
Spindt tips, which are made from an electron-emissive metal, such
as molybdenum. These electron emitters are susceptible to surface
contamination by oxygen-containing and carbon-containing species.
The surface oxygen and carbon have deleterious effects on the
electron emission properties of the electron emitters. In
particular, the presence of oxygen and carbon at the emissive
surface increases the surface work function of the electron
emitters. That is, a bigger electric field is required to extract
electrons therefrom due to the contamination. Surface contaminants
also result in emission current instability and reduced device
lifetime.
Metal field emission tips have been employed in field emission
electron and ion microscopy, scanning tunneling microscopy, etc. It
is known to remove surface contaminants from electron emitters in
these microscopy systems by employing high temperature (greater
than 2000.degree.K) flashing. However, field emission arrays often
include glass substrates upon which the electron emitters are
formed. These glass substrates have temperature tolerances upwards
of 700-800.degree.K. Thus, high temperature cleaning procedures
cannot be used for decontaminating field emission electron emitters
formed on glass substrates.
Furthermore, the contamination of field emission electron emitters
occurs throughout the life of the field emission device.
Contaminant gaseous species are introduced into the vacuum of the
field emission device by outgassing from surfaces, by
electron-stimulated desorption from the phosphors and other
surfaces that are exposed to field emitted electrons, by small
leaks in the packaging elements, etc.
In order to maintain constant emission characteristics over the
life of the device, it is desirable that emitter surface
contaminants be removed throughout the life of the device. It is
also desirable that this cleaning process be continuous over the
life of the device or be performed periodically at a frequency that
is sufficient to prevent appreciable deterioration of emission
characteristics. However, field emission devices typically have no
convenient means for introducing cleaning agents into the device
subsequent to the vacuum sealing of the device package.
Accordingly, there exists a need for a method for in situ
decontamination of electron emitters in field emission devices.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a first embodiment of a field
emission device useful for performing steps of a method in
accordance with the invention;
FIG. 2 is a cross-sectional view of a second embodiment of a field
emission device useful for performing steps of a method in
accordance with the invention;
FIG. 3 is a cross-sectional view of a third embodiment of a field
emission device useful for performing steps of a method in
accordance with the invention and includes a block diagram of a
configuration for controlling the rate of hydrogen evolution from a
hydrogen source in accordance with the invention;
FIG. 4 is a cross-sectional view of a fourth embodiment of a field
emission device useful for performing steps of a method in
accordance with the invention and includes a block digram of a
configuration for controlling the rate of hydrogen evolution from a
hydrogen source; and
FIG. 5 is a cross-sectional view of a fifth embodiment of a field
emission device useful for performing steps of a method in
accordance with the invention.
It will be appreciated that for simplicity and clarity of
illustration, elements shown in the FIGURES have not necessarily
been drawn to scale. For example, the dimensions of some of the
elements are exaggerated relative to each other. Further, where
considered appropriate, reference numerals have been repeated among
the FIGURES to indicate corresponding elements.
DESCRIPTION
The invention is for a method for in situ cleaning of electron
emitters in a field emission device. The method of the invention
includes the steps of providing hydrogen gas within the device
during the operational life of the device and, thereafter, causing
the electron emitters of the field emission device to emit
electrons. These electrons interact with the hydrogen gas to form
hydrogen free radicals, which are useful for cleaning and
conditioning the surfaces of the electron emitters. The
decontaminated emissive surfaces provide stable emission current
during the operational life of the device. The method of the
invention further includes the step of providing within the field
emission device hydrogen gas at a rate/frequency sufficient to
maintain the surfaces of the electron emitters free of contaminants
and thereby maintain stable electron emission over the life of the
device.
The field emission devices described herein are directed to field
emission display devices having triode configurations and employing
Spindt tip electron emitters. However, the scope of the invention
is not intended to be limited to display devices, to devices having
a triode configuration, nor to devices having Spindt tip electron
emitters. In general, the invention can be embodied in a vacuum
device that employs field emission electron emitters, such as
Spindt tips, edge emitters, wedge emitters, surface conduction
emitters, and the like, which are made from a material that can be
cleaned/decontaminated using hydrogen free-radicals. Also, the
invention can be embodied in a field emission device having a diode
configuration or a configuration having greater than three
electrodes.
FIG. 1 is a cross-sectional view of a first embodiment of a field
emission device (FED) 100 useful for performing steps of a method
for in situ cleaning of a plurality of electron emitters 126 in
accordance with the invention. FED 100 is an operational device.
That is, FED 100 has the operational configuration that is intended
for its functional life. FED 100 is evacuated, and the device
package components are hermetically sealed. The method of the
invention is useful for providing hydrogen gas within FED 100 at a
time during the operational life of FED 100, subsequent to its
evacuation and sealing. This allows for in situ cleaning of
electron emitters 126.
FED 100 includes a cathode plate 110, which is spaced from an anode
plate 112 to define an interspace region 114 therebetween. Cathode
plate 110 includes a plurality of electron emitters 126. In
general, during the operation of FED 100, electrons, indicated by a
dashed line 134 in FIG. 1, are emitted by electron emitters 126 and
are subsequently collected at anode plate 112.
Cathode plate 110 includes a substrate 116, which can be made from
glass, silicon, or some other hard, dielectric material. Upon
substrate 116 is disposed a plurality of cathodes 118, which are
electrodes made from a conductor, such as molybdenum, aluminum, and
the like. A dielectric layer 120 is disposed on cathodes 118 and
defines a plurality of emitter wells 124. Electron emitters 126 are
disposed one each in emitter wells 124. In the embodiment of FIG.
1, electron emitters 126 include Spindt tips. Electron emitters 126
are made from a field emissive material. Exemplary field emissive
materials include molybdenum, niobium, hafnium, tungsten, iridium,
silicon, diamond-like carbon, and the like. In general, the field
emissive material can be induced to emit electrons by the
application of an electric field of appropriate strength. Also, the
field emissive material can be conditioned/cleaned using hydrogen
free radicals, which include atomic hydrogen and hydrogen ions.
A plurality of gate extraction electrodes 122 is configured upon
dielectric layer 120 for selectively addressing electron emitters
126. Gate extraction electrodes 122 are made from a conductive
material, such as molybdenum, aluminum, and the like. Methods for
fabricating cathode plate 110 are known to one skilled in the
art.
Anode plate 112 includes a transparent substrate 128 made from a
solid, transparent material, such as a glass. An anode 130 is
formed on transparent substrate 128. Anode 130 is made from a
transparent, conductive material, such as indium tin oxide. Anode
plate 112 further includes a plurality of phosphors 132, which are
made from a cathodoluminescent material.
Between cathode plate 110 and anode plate 112, at their
peripheries, is disposed a frame 136, which provides standoff
therebetween. Frame 136 can be made from a glass and is affixed to
cathode plate 110 with a sealant 138. Sealant 138 can be a frit
sealant, indium metal, tin, indium tin alloys, other low melting
point metals, and the like. Cathode plate 110, anode plate 112, and
frame 136 define a device package.
A hydrogen-selective membrane 140 is disposed within a hole 144
defined by frame 136 and anode plate 112. Hydrogen-selective
membrane 140 is made from a refractory metal, such as palladium,
nickel, a palladium alloy, a nickel alloy, and the like, which is
selectively permeable with respect to hydrogen. Preferably,
hydrogen-selective membrane 140 is made from palladium.
Hydrogen-selective membrane 140 has a thickness, in the direction
of hydrogen diffusion, within a range of 50-500 micrometers. Under
the appropriate conditions of temperature and pressure, hydrogen
gas is capable of selectively diffusing through hydrogen-selective
membrane 140.
FED 100 can be fabricated by first silk-screening sealant 138 onto
transparent substrate 128 at the periphery thereof. Then,
hydrogen-selective membrane 140 is disposed on sealant 138.
Refractory metal membranes, having thicknesses greater than about
10 micrometers, are available commercially. Such a refractory metal
membrane can be cut into a suitable shape to form
hydrogen-selective membrane 140. The structure is then heated to
affix the refractory metal to sealant 138.
Anode plate 112, having hydrogen-selective membrane 140 formed
thereon, is assembled with cathode plate 110, having frame 136
affixed thereto, in a vacuum environment, so that a vacuum is
realized within interspace region 114. As illustrated in FIG. 1,
hydrogen-selective membrane 140 is thus disposed in communication
with interspace region 114. That is, hydrogen gas, which is
indicated by an arrow 142 in FIG. 1, that diffuses through
hydrogen-selective membrane 140 can subsequently travel into
interspace region 114.
Subsequent to the steps of sealing the elements of FED 100 and
establishing a vacuum environment therein, the following steps are
used to achieve in situ feeding of hydrogen gas into interspace
region 114 and in situ cleaning of electron emitters 126, in
accordance with the invention. First, FED 100 is placed in an oven
having a hydrogen atmosphere. The hydrogen atmosphere within the
oven has a hydrogen partial pressure within a range of milli-Torr
to several atmospheres. Then, the temperature within the oven is
elevated to within a range of about 273-450.degree.K. In general,
the temperature and partial pressure of hydrogen within the
hydrogen atmosphere are selected to promote diffusion of hydrogen
gas through hydrogen-selective membrane 140. The hydrogen gas can
be controllably provided by manipulating the temperature and the
hydrogen pressure external to FED 100 during the diffusion
step.
The diffusion of hydrogen into interspace region 114 is performed
for a period of time sufficient to provide within interspace region
114 a partial pressure of hydrogen useful for cleaning electron
emitters 126. In accordance with the invention, a partial pressure
of hydrogen is achieved within FED 100 that is preferably greater
than 10.sup.-8 Torr, most preferably within a range of 10.sup.-8
-10.sup.-5 Torr.
The hydrogen content can be determined by measuring the total
pressure within FED 100 prior to the addition of hydrogen and
thereafter measuring the total pressure within FED 100 after the
addition of hydrogen. If these two measurements are taken at the
same temperature, the final hydrogen partial pressure can be
derived therefrom by, for example, using the ideal gas law.
In general, clean electron emitters 126 ameliorate the fluctuations
in the emission current for a given set of conditions, including
operating voltages and temperature. Thus, the level of
contamination of electron emitters 126 can be deduced from measured
fluctuations in the emission current. Contamination of electron
emitters 126 is also reflected by a reduction in the emission
current for a given set of emission parameters. In accordance with
the invention, a partial pressure of hydrogen is established within
interspace region 114 that provides stabilized, constant emission
current having fluctuations within a tolerable range. For example,
it may be desirable to maintain current fluctuations of less than
0.5% per hour for a given set of conditions.
Subsequent to the step of providing a partial pressure of hydrogen
gas within interspace region 114 useful for cleaning electron
emitters 126, electron emitters 126 are activated to emit
electrons. Electron emission is realized by applying the
appropriate potentials to cathodes 118 and gate extraction
electrodes 122, as is known to one skilled in the art. The emitted
electrons are then attracted toward anode 130 by applying thereto
an appropriate potential. As they travel across interspace region
114, the emitted electrons dissociate and ionize the hydrogen
molecules present therein, thereby forming hydrogen free radicals
within interspace region 114.
The hydrogen free-radicals, which include hydrogen ions and
energetic neutral hydrogen atoms, react with the surfaces of
electron emitters 126, which include surface contaminants, to form
volatile hydrides. Exemplary volatile hydrides that may be produced
include: H.sub.2 O, MoH.sub.x.sup.+ (x=1-3), MoOH.sup.+, OH.sup.+,
OH, H.sup.+, CH.sub.x.sup.+ , (x=1-4), and the like. These volatile
hydrides are then removed from interspace region 114 by gettering
material (not shown) present within FED 100.
It is desired to be understood that the cleaning process is not
limited to the removal of contaminants in the form of hydrides
alone. The hydrogen free radicals are also capable of catalyzing
surface chemical reactions, which produce volatile products that do
not include hydrides and which effectively remove surface
contaminants.
In accordance with the invention, the steps for cleaning and
conditioning electron emitters 126 can be performed shortly after
sealing of the device package to remove surface contaminants,
native oxides, and process residues. These steps can also be
performed after a period of use of FED 100, thereby reconditioning
electron emitters 126 and removing surface contaminants accumulated
during the operation of FED 100. This cleaning procedure is
preferably performed periodically over the life of FED 100, at a
frequency sufficient to maintain clean electron emitters 126. In
this manner, stable, constant electron emission is realized over
the life of FED 100.
In general, the means for in situ feeding of hydrogen is disposed
in communication with the interspace region of the device package.
In the embodiment having a hydrogen-selective membrane, the
hydrogen-selective membrane is configured in registration with a
hole/gap defined by the device package. Under appropriate
conditions of pressure and temperature, this configuration allows
hydrogen gas to diffuse from a hydrogen atmosphere external to the
field emission device, through the hydrogen-selective membrane, and
into the interior of the field emission device.
FIG. 2 is a cross-sectional view of a second embodiment of a field
emission device (FED) 200 useful for performing steps of a method
in accordance with the invention. FED 200 includes a
hydrogen-selective membrane 240, which is disposed in registration
with a hole 244 defined by the device package. Hole 244 is defined
by a transparent substrate 228 of an anode plate 212. Transparent
substrate 228 is made from a hard, transparent material, such as a
glass, and has affixed thereto an anode 230 and a plurality of
phosphors 232. Hydrogen-selective membrane 240 overlies hole 244.
Hydrogen-selective membrane 240 includes a membrane made from a
refractory metal such as palladium, nickel, and the like, which is
selectively permeable to hydrogen. The thickness of
hydrogen-selective membrane is preferably within a range of 50-500
micrometers.
FED 200 is fabricated by first making a cathode plate 210, in a
manner similar to that described with reference to FIG. 1. Cathode
plate 210 includes a plurality of cathodes 218, a plurality of
electron emitters 226, and a plurality of gate extraction
electrodes 222. A frame 236 is attached to the periphery of cathode
plate 210 by using a frit sealant (not shown). Anode plate 212 is
attached to frame 236 to define an interspace region 214. The step
of attaching anode plate 212 can be performed in air because,
subsequent to the sealing process, interspace region 214 can be
evacuated through hole 244 using a vacuum pump, as is known to one
skilled in the art.
Hydrogen-selective membrane 240 is affixed to anode plate 212 by
first providing a ring 246 made from an alloy having thermal
expansion characteristics that match those of transparent substrate
228. Hydrogen-selective membrane 240 is brazed to ring 246, so that
it covers the hole defined by ring 246. Then the hole defined by
ring 246 is positioned in registration with hole 244 of transparent
substrate 228. Ring 246 is attached to transparent substrate 228
using a frit sealant 248. The step of attaching ring 246, having
hydrogen-selective membrane 240 affixed thereto, to transparent
substrate 228 is performed subsequent to the evacuation of the
device package.
Subsequent to the step of attaching hydrogen-selective membrane 240
to the device package, a hydrogen partial pressure is established
within FED 200, in a manner similar to that described with
reference to FIG. 1. Under appropriate conditions of temperature
and pressure, hydrogen gas, which is indicated by an arrow 242 in
FIG. 2, is diffused through hydrogen-selective membrane 240 and
travels into interspace region 214. Within interspace region 214,
the hydrogen gas is converted into hydrogen free radicals by
electrons, which are indicated by a dashed line 234 in FIG. 2, that
are emitted by electron emitters 226.
FIG. 3 is a cross-sectional view of a third embodiment of a field
emission device (FED) 300 useful for performing steps of a method
in accordance with the invention, and includes a block diagram of
means for controlling the rate of hydrogen evolution from a
hydrogen source 340. Similar to FED 100 and FED 200 of FIGS. 1 and
2, respectively, FED 300 is an operational device. That is, FED 300
has the operational configuration that is intended for its
functional life. FED 300 is evacuated, and the device package
components are hermetically sealed. The method of the invention is
useful for providing hydrogen gas within FED 300 at a time during
the operational life of FED 300, subsequent to its evacuation and
sealing. This allows for in situ cleaning of a plurality of
electron emitters 326.
In accordance with the invention, a method for in situ cleaning of
electron emitters 326 includes the steps of providing hydrogen
source 340 within FED 300, releasing hydrogen gas from hydrogen
source 340, and, thereafter, forming hydrogen free radicals from
the hydrogen gas. The components of the block diagram of FIG. 3 are
useful for controllably releasing the hydrogen gas from hydrogen
source 340. In the embodiment of FIG. 3, the step of controllably
releasing hydrogen gas from hydrogen source 340 includes the step
of releasing hydrogen gas from hydrogen source 340 in response to a
drop below a predetermined value of a test emission current 358
measured at a test electron emitter 359 in FED 300.
FED 300 has a cathode plate 310 and an anode plate 312, which
define an interspace region 314. FED 300 further includes hydrogen
source 340, which is disposed within interspace region 314.
Hydrogen source 340 includes a solid member made from a refractory
metal, such as palladium, nickel, a palladium alloy, a nickel
alloy, and the like. Preferably, hydrogen source 340 is made from
palladium. Hydrogen source 340 is secured to one of the surfaces
defining interspace region 314 by a convenient method, such as by
using a frit sealant or mechanical means.
Hydrogen source 340 contains hydrogen. In accordance with the
invention, hydrogen is provided within hydrogen source 340 by first
placing the metallic member in an oven having a hydrogen
atmosphere. The temperature in the oven is elevated to induce the
diffusion of hydrogen gas into the metallic member. After a
sufficient amount of hydrogen has been diffused into the metallic
member, the metallic member is cooled, thereby entrapping the
hydrogen contained therein.
In accordance with the invention, electron emitters 326 are cleaned
and conditioned by first controllably releasing hydrogen gas, which
is indicated by an arrow 342 in FIG. 3, from hydrogen source 340.
The rate/frequency of hydrogen evolution from hydrogen source 340
is controlled so as to provide within interspace region 314 a
partial pressure of hydrogen that is useful for maintaining a
stable emission current. A dashed line 334 in FIG. 3 indicates the
emission current.
The step of releasing hydrogen gas from hydrogen source 340
includes the step of heating hydrogen source 340. Hydrogen source
340 can be heated by passing a current directly through hydrogen
source 340. Alternatively, hydrogen source 340 can be heated by
providing a heating element, such as a resistive wire, and
providing thermal contact between hydrogen source 340 and the
heating element. Subsequent to the step of heating hydrogen source
340, hydrogen free radicals are produced from the hydrogen gas, in
the manner described with reference to FIGS. 1 and 2. Hydrogen
source 340 can also be heated by using an infrared laser.
Illustrated in FIG. 3 is a block diagram of a control system useful
for controlling the rate of hydrogen evolution from hydrogen source
340 in accordance with the method of the invention. The control
system includes a switching circuit 354, a controller 356, a
temperature measurement device 366, and a current measurement
device 362.
Controller 356 controls test emission current 358 that is emitted
by test electron emitter 359. The characteristics of test emission
current 358 are representative of the characteristics of the
emission currents from the remainder of electron emitters 326.
Controller 356 controls test emission current 358 by manipulating
the rate of hydrogen evolution from hydrogen source 340 in response
to a first signal 364 from current measurement device 362 and a
second signal 368 from temperature measurement device 366.
A current measurement electrode 360 is configured on anode plate
312 to receive test emission current 358. Current measurement
device 362 is connected to current measurement electrode 360 for
measuring test emission current 358. Current measurement device 362
transmits first signal 364, which is related to test emission
current 358, to a first input terminal 361 of controller 356.
Temperature measurement device 366 measures a temperature within
interspace region 314 and transmits second signal 368, which is
related to the temperature, to a second input terminal 363 of
controller 356. The value of the emission current is dependent, in
part, upon temperature. Controller 356 corrects for this
temperature dependence when determining the status of the emission
current. When the corrected value of the emission current drops
below a predetermined level, the controller transmits a control
signal 357 to a first input terminal 353 of switching circuit
354.
Switching circuit 354 is responsive to control signal 357.
Switching circuit 354 has an output that is connected to hydrogen
source 340 for transmitting an activation current 350 thereto. In
general, switching circuit 354 transmits activation current 350 to
hydrogen source 340 when the corrected emission current drops below
a predetermined value due to surface contamination of electron
emitters 326. In the embodiment of FIG. 3, a voltage source 352 is
connected to a second input terminal 351 of switching circuit 354.
Voltage source 352 can be included in the power supply of FED
300.
Due to the heating of hydrogen source 340, the temperature within
FED 300 may increase. It is desired to maintain the temperature
within FED 300 below that which results in an excessive,
catastrophic emission current at electron emitters 326. Controller
356 is designed to cease heating hydrogen source 340 when the
temperature measured by temperature measurement device 366 reaches
an upper limit. In this manner, the emission current is prevented
from attaining a catastrophic level due to overheating within FED
300 caused by the heating of hydrogen source 340.
FIG. 4 is a cross-sectional view of a fourth embodiment of a field
emission device (FED) 400 useful for performing steps of a method
in accordance with the invention, and includes a block diagram of
means for controlling the rate of hydrogen evolution from hydrogen
source 340. In the method for in situ cleaning of electron emitters
326 of FED 400, the step of controllably releasing hydrogen gas
from hydrogen source 340 includes the step of releasing hydrogen
gas in response to the activation of FED 400. In this manner, the
rate of cleaning of electron emitters 326 is responsive to the
extent of use of FED 400.
FED 400 includes anode plate 112 and cathode plate 310, which
define an interspace region 414. In the embodiment of FIG. 4, the
system for controlling the rate of hydrogen evolution from hydrogen
source 340 includes a current source 474 and an N-counter circuit
472.
FED 400 has a start-up circuit 470, which initially activates the
device. Start-up circuit 470 is coupled to cathode plate 310 and
anode plate 112 (connections not shown) and provides the proper
operating voltage for powering FED 400. When start-up circuit 470
is activated, it transmits a start-up signal 480 to an input
terminal 476 of N-counter circuit 472. Start-up signal 480 triggers
a counter. When the counter reaches N, N-counter circuit 472
transmits from an output terminal 477 an activation signal 478.
Activation signal 478 is received at an input terminal 471 of
current source 474.
Current source 474 has an output terminal 473 that is connected to
hydrogen source 340. Upon receipt of activation signal 478, current
source 474 transmits an activation current 475 to hydrogen source
340, resulting in evolution of hydrogen gas from hydrogen source
340.
The amount of current sent to hydrogen source 340 each time
N-counter reaches N and the value of N depend upon factors such as
the size of FED 400 and the anticipated extent of contamination
during a given period of use of FED 400. The latter factor depends
in part upon the nature of the materials present within FED 400.
For example, different materials may generate contaminants at
different rates.
In accordance with the invention, the step of controllably
releasing hydrogen gas from a hydrogen source can include the step
of periodically releasing hydrogen gas from the hydrogen source. A
field emission device useful for performing periodic hydrogen
emissions has a configuration similar to that depicted in FIG. 4
and includes a control system having a timer circuit. In this
embodiment, a current source is connected to the hydrogen source;
however, the timer circuit is substituted for the N-counter
circuit. The timer circuit generates a periodic activation signal,
which is transmitted to the current source. In this manner, a
predetermined amount of current can be periodically transmitted to
the hydrogen source at predetermined intervals. For example,
hydrogen evolution can be provided once per month using this
configuration.
The step of maintaining within the field emission device a partial
pressure of hydrogen gas sufficient to maintain stable electron
emission can also include the step of maintaining within the field
emission device a partial pressure of hydrogen greater than
10.sup.-8 Torr, preferably within a range of 10.sup.-8 -10.sup.-5
Torr. A field emission device useful for performing this step
includes a feedback control system, similar to that of FIG. 3, for
controlling the hydrogen partial pressure. For example, this
feedback control system can include a device for measuring the
total pressure within the device. A change in the total pressure
can be attributable to a change in the partial pressure of
hydrogen. The pressure measurement device transmits a signal, which
is related to total pressure, to a controller. When the pressure
drops below a predetermined value, the controller transmits a
control signal to a switch or current source, to generate an
activation current. The activation current is transmitted to the
hydrogen source to cause hydrogen evolution therefrom.
FIG. 5 is a cross-sectional view of a fifth embodiment of a field
emission device (FED) 500 useful for performing steps of a method
in accordance with the invention. The introduction of hydrogen,
which is indicated by an arrow 542, into an interspace region 514
of FED 500 is realized by an electron-stimulated hydrogen
desorption process. Specifically, the step of releasing hydrogen
gas from a hydrogen source 540 includes the step of impinging
electrons, which are indicated by a dashed line 590, onto hydrogen
source 540. The rate of hydrogen evolution depends upon the density
and energy of the impinging electron beam, and no thermal heating
of hydrogen source 540 is required.
In the embodiment of FIG. 5, an activation electron emitter 585
provides the impinging electron beam. Hydrogen source 540 opposes
activation electron emitter 585. Hydrogen source 540 is made in the
manner described with reference to hydrogen source 340 of FIGS. 3
and 4. A cathode plate 510 includes activation electron emitter
585, which is one of a plurality of electron emitters 526 disposed
within emitter wells defined by a dielectric layer 520. Electron
emitters 526 are connected to a plurality of cathodes 518, which
are disposed on a substrate 516.
An activation gate extraction electrode 587 is disposed proximate
to activation electron emitter 585 and is coupled to a voltage
source 592. Activation gate extraction electrode 587 is controlled
independently from a plurality of gate extraction electrodes 522,
which are used to selectively address those of electron emitters
526 that oppose a plurality of phosphors 532.
Selectively addressing activation electron emitter 585 controllably
provides the hydrogen gas. Voltage source 592 is used to
selectively apply an extraction voltage at activation gate
extraction electrode 587. When hydrogen evolution from hydrogen
source 540 is desired, voltage source 592 is used to apply the
extraction voltage to activation gate extraction electrode 587,
thereby realizing electron emission from activation electron
emitter 585. When no hydrogen evolution from hydrogen source 540 is
desired, voltage source 592 is used to apply a voltage that does
not result in electron emission from activation electron emitter
585. The output voltage of voltage source 592 can be manipulated
using one of a number of useful control methods, such as those
described with reference to FIGS. 3 and 4.
An electron-attracting voltage is provided at hydrogen source 540
by a voltage source (not shown), so that the electrons from
activation electron emitter 585 are attracted to and collected at
hydrogen source 540. In the embodiment of FIG. 5, hydrogen source
540 is disposed on an anode plate 512. Anode plate 512 includes a
transparent substrate 528, upon which is formed an anode 530.
Hydrogen source 540 is connected to anode 530, to which the
electron-attracting voltage is applied. Phosphors 532 are also
configured on anode 530. The electrons collected at hydrogen source
540 stimulate hydrogen evolution therefrom. The hydrogen thus
evolved is then ionized by electrons within interspace region 514,
including the electrons, which are generally indicated by a dashed
line 534, directed toward phosphors 532.
In an alternative embodiment, the hydrogen source is not coupled to
the anode. Rather, the hydrogen source is coupled to an independent
voltage source, so that the voltage at the hydrogen source can be
manipulated independently from the voltage at the phosphors. In
this particular embodiment, the electrons for use for hydrogen
evolution can be provided by any of the electron emitters within
the device. The emitted electrons are directed toward the hydrogen
source by selectively biasing it to attract the electrons. For
example, subsequent to the sealing and evacuation of the device,
some or all of the electron emitters are caused to emit electrons.
Simultaneously, a positive, attracting voltage is selectively
applied to the hydrogen source. After the decontamination steps are
completed, the positive, attracting voltage is removed from the
hydrogen source. Any subsequently emitted electrons can be directed
toward the phosphors by selectively applying a positive, attracting
voltage to the phosphors.
In summary, the invention is for a method for in situ cleaning of
electron emitters in a field emission device. The method of the
invention includes steps for controllably providing hydrogen gas
within the field emission device and, thereafter, forming hydrogen
free radicals from the hydrogen gas. The hydrogen free radicals are
useful for cleaning the electron emitters. The steps of the method
of the invention can be performed at any time subsequent to the
vacuum sealing of the device package. Further, the hydrogen gas is
controllably introduced at a rate/frequency sufficient to remove
surface contaminants and maintain clean electron emitters, thereby
realizing stable electron emission over the life of the device.
While we have shown and described specific embodiments of the
present invention, further modifications and improvements will
occur to those skilled in the art. We desire it to be understood,
therefore, that this invention is not limited to the particular
forms shown, and we intend in the appended claims to cover all
modifications that do not depart from the spirit and scope of this
invention.
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